This invention relates to viscometers used to measure or characterize the stress needed to shear a fluid at a given rate. In particular, this invention relates to viscometers for continuously monitoring changes in the viscosity of fluids used in or produced by a device or process including low-viscosity fluids such as engine lubricants, by monitoring the torque required to achieve differential rotation between two elements defining a flow path for the fluid there between. Such viscometers may be used for example in on-board systems to maintain the quality of engine lubricants which is essential to the proper operation and long service life of internal combustion engines or other equipment.
One common form of viscometer comprises two coaxial cylinders (cylinder-in-cylinder) which are rotated relative to one another while measuring, either visually or electronically, the torque, or torque equivalent, required to achieve a differential rotation speed. The flow characteristics of the fluid can be determined by interposing the fluid in an annular gap between the cylinders and for a known differential rotational speed, measuring the torque, or torque equivalent. By factoring in the physical dimensions and the drag associated with bearings or seals of the viscometer that can affect torque measurement, the viscosity of the fluid can be calculated for a particular shear rate. Typically, a viscometer is driven at a single speed and the viscosity calculated at a single shear rate to allow relative comparison of fluids. However, if desired the viscometer can also be used to more fully characterize a fluid, by measuring torque over a range of differential rotational speeds.
In certain applications, viscometers are used to continuously monitor a fluid used in or produced by a device or process. The fluids can be either totally liquid or a liquid containing particulate. One method for using known coaxial cylinder viscometers in these applications is to put the viscometers in line with the fluid flow. Problems with this method include the complexity of designing the viscometers into the flow circuit, the difficulty in replacing components of the viscometers should failure occur, and accuracy issues should the fluid flow past the viscometers vary from a constant rate.
One way of overcoming some of the problems associated with mounting cylinder-in-cylinder viscometers in line with the flow path is to mount the viscometers outside the main flow path. In this arrangement, the outer cylinder of the viscometers is capped to form a cup-like structure with the inner cylinder or bob inside the cup. This allows the drive for the differential rotation to be mounted quite close to the rotating elements for a more compact design and also allows maintenance issues to be more easily addressed.
A problem with prior bob-in-cup viscometers used to continuously monitor a fluid used in or produced by a device or process is that a pump or other hardware is needed to control the fluid flow through the viscometers, which adds to the cost and complexity to using the viscometers. Another problem with prior bob-in-cup viscometers is that, when used to accurately measure low viscosity fluids containing particulate, particulate settling can occur resulting in inaccurate viscosity calculation. Thus, careful placement of prior bob-in-cup viscometers is critical to proper operation. Also, such viscometers are potentially subject to a number of possible sources of error due to unwanted friction and/or drag effects.
The present invention overcomes the above-noted and other shortcomings of prior bob-in-cup viscometers by providing a relatively simple way of continuously monitoring fluid viscosity without the cost and complexity of a pump or other hardware to maintain flow through the viscometers, and without the placement issues normally needed to prevent particulate settling when measuring particulate-containing low-viscosity fluids.
In accordance with one aspect of the invention, the viscometers are self pumping in order to maintain controlled fluid flow through the sections of the viscometers where the fluid flow properties are measured, essentially independent of flow rate of the fluid through its primary flow path. The self-pumping character of the viscometers is also a benefit in preventing particulate settling when used to accurately measure relatively low viscosity (e.g., 1 to 100 cSt.) fluids that may contain finely, relatively well-dispersed solids.
In accordance with another aspect of the invention, the viscometer bobs and cups are designed such that relative rotation between the two elements urges the fluid to flow through the viscometers due to a pressure differential caused by the rotation.
In accordance with another aspect of the invention, the flow through the viscometers is both controlled and sufficient to minimize or eliminate clogging due to any particulate settling from the fluid.
In accordance with another aspect of the invention, in one embodiment, the bob comprises a hollow cylinder closed at one end adjacent the closed end of the cup and open at the other end. Extending through the wall of the bob at a location near its closed end and facing a continuous wall of the cup are a plurality of discrete circumferentially spaced openings. Differential bob/cup rotation urges fluid from a volume outside the bob through the bob and out through the discrete openings in the bob wall for passage through an annular gap between the bob and cup and into a volume outside the cup.
In accordance with another aspect of the invention, in another embodiment, a plurality of discrete circumferentially spaced openings are provided in the wall of the cup near the closed end of the cup facing a continuous wall of the bob. Differential bob/cup rotation urges fluid from the volume outside the bob, through the annular gap between the cup and bob and out through the discrete openings of the cup wall to a volume outside the cup.
In accordance with another aspect of the invention, in another embodiment, the wall of the bob has discrete circumferentially spaced openings near one end facing a continuous wall of the cup, and the cup has discrete circumferentially spaced openings facing a continuous wall of the bob near the end of the bob that is opposite the end of the bob containing discrete openings. Differential bob/cup rotation urges fluid from a volume outside the bob through the discrete wall openings of the bob and annular gap between the bob and cup and out through the discrete openings in the cup to a volume outside the cup.
In accordance with another aspect of the invention, in another embodiment, the bob is a cylinder of finite side wall thickness open at both ends. Also, one of the open ends is spaced from the closed end of the cup an axial distance of between one half to five times the radial separation between the bob and cup, whereby differential bob/cup rotation urges fluid from a volume outside the bob through the bob, the separation between the end of the bob and closed end of the cup, and the annular gap between the bob and cup and into a volume outside the cup.
In accordance with another aspect of the invention, in another embodiment, a series of alternate coaxial cylinders of finite wall thickness are alternately supported by a pair of axially spaced end plates to provide alternate coaxial bobs and cups. One end plate has a central opening providing fluid communication between a volume outside the end plate and the center cylinder. Discrete circumferentially spaced openings are provided through the cylindrical wall of at least one bob/cup near its open end facing a continuous cylindrical wall of an adjacent cup/bob. Differential rotation of the end plates urges fluid from a volume outside the viscometer through the center cylinder and separations between the bob/cup cylinders and opposed end plates and through the circumferentially spaced openings in at least one bob/cup cylinder and the annular gaps between adjacent bob/cup cylinders and out through the annular gap between the last two bob/cup cylinders into the volume outside the viscometer.
In accordance with another aspect of the invention, in another embodiment, the separation between the end of at least one of a plurality of coaxial bob/cup cylinders and the opposed end plate is between one half to five times the annular gap between adjacent bob/cup cylinders. Differential rotation of the end plates urges fluid from a volume outside the viscometer through the center bob and separations between the bob/cup cylinders and opposed end plates and through the annular gaps between adjacent bob/cup cylinders and out through the annular gap between the last two bob/cup cylinders into the volume outside the viscometer.
In accordance with another aspect of the invention, in another embodiment, the bob and cup are axially symmetric but non-cylindrical. Also, the bob has a coaxial bore extending all the way through the bob, and an annular gap is provided between the bob and cup that either remains the same or increases as a function of radius from the common axis of the bob and cup. Differential bob/cup rotation urges fluid from a volume outside the bob through the coaxial bore of the bob, through the gap between the bob and cup and into the volume outside the cup.
In accordance with another aspect of the invention, a magnetic drive coupling is provided between the rotating element of the viscometer and the viscometer drive motor.
In accordance with another aspect of the invention, in one embodiment, the rotating element is the driven magnet of the magnetic drive coupling and is surrounded by the driving magnet, allowing the rotating element to self-locate centrally in the magnetic field of the driving magnet, thus eliminating the need for end thrust location of the rotating element, which is a possible source of error due to friction on the thrust faces.
In accordance with another aspect of the invention, the rotating element is mounted on a hollow shaft which permits fluid from a volume outside the rotating element to pass through the rotating element into a separation between the end of the rotating element and the closed end of a relatively fixed cup. Rotation of the rotating element within the cup urges fluid from a volume outside the rotating element through the rotating element and separation between the end of the rotating element and closed end of the cup and through the annular gap between the cup and rotating element and into a volume outside the cup.
In accordance with another aspect of the invention, the portion of the viscometer housing carrying bearing bushes for the rotating shaft is provided with radial holes to reduce the friction effect caused by fluid between the rotating shaft and housing in order to reduce unwanted drag effects.
In accordance with another aspect of the invention, in another embodiment, the driven magnet is mounted on the bottom side of the viscometer bob and is polarized north and south from one side to the other for magnetic coupling with a driving magnet on the rotor shaft.
In accordance with another aspect of the invention, a bob shaft is pressed into an axial hole in the bob and has radiuses at both ends slightly smaller than half ball radiuses in insert bearings in which the bob shaft ends are received to cause the bob to move like a gyro with little effort required.
In accordance with another aspect of the invention, end play between the bob shaft and insert bearings is preferably no more than 0.010 inch, whereby the viscometer may operate in any position.
In accordance with another aspect of the invention, a plurality of circumferentially spaced slots are provided in the side of the cup in line with the cup bottom to allow debris and sediment entering the annular gap between the cup and bob to exit the cup and allow free flow of fluid through such annular gap.
To the accomplishment of the foregoing and related ends, the invention, then, comprises the features hereinafter fully described and particularly pointed out in the claims, the following description and the annexed drawings setting forth in detail certain illustrative embodiments of the invention, these being indicative, however, of but a few of the various ways in which the principles of the invention may be employed.